The present invention relates to methods and apparatus for increasing facsimile call success rates. More particularly, the invention relates to facsimile call transmissions in the presence of long delays and channel errors such as those present in wireless access systems, e.g., terrestrial cellular, fixed wireless, and geostationary mobile satellite systems.
Conventional facsimile machines and compatible terminals are designed for communication over a public service telephone network. Standard protocols have been adopted for communication between calling and called facsimile terminals. Examples of such protocols are those defined by the International Telegraph and Telephone Consultative Committee (CCITT) under Recommendations T.3 and T.4, known respectively as the Group 2 and Group 3 facsimile protocols. Recommendation T.30 defines a protocol for Group 2 and 3 facsimile equipment for communication over a telephone network.
Previous techniques such as flag-stuffing require explicit knowledge of delays in the systems for it to be effective, since International Telecommunications Union standard ITU-T T.30 limits the duration of High Level Data Link Control (HDLC) frames, which includes HDLC flags. Knowledge of delay would assist the flag-stuffing technique to determine the time at which flag-stuffing should start so as not to violate the T.30 frame duration constraints.
An alternative means for communication between facsimile terminals has been proposed, in which each facsimile terminal is connected to a facsimile interface unit for communication via a public service telephone network to demodulate the signal that was modulated by the facsimile machine and transmit the demodulated data on digital satellite/cellular links, and modulate the data received over these links towards the end facsimile machines. This has the advantage of conserving band-width on bandwidth-limited satellite/cellular networks, since otherwise the modulated signal would have to be carried at the rate of 64 kbps. Thus, encoded data suitable for transmission over a digital network is provided, for example a digital satellite link or a cellular telephone system. However, the introduction of the Facsimile Interface Units (FIU) adds processing delay to the long propagation delays in satellite/cellular networks that might further decrease call success rates. One example where the processing delay becomes a significant source is the facsimile demodremod unit associated with Digital Circuit Multiplication Equipments (DCME), where processing/buffering delay can be as high as 300 ms. This increased delay may cause failures in communication between the calling and called facsimile terminals. A standard for overcoming this increased delay is not provided under the Group 3 fax protocol.
The document WO 92/02100 discloses a facsimile interface unit which automatically sends a xe2x80x9ccommand repeatxe2x80x9d signal to a facsimile terminal on receipt of a command therefrom, in order to allow more time for a response signal to be received.
The INMARSAT-B (TM) System Definition Manual, Issue 2, dated September 1989, proposes programming a facsimile interface unit to send a sequence of flags to a facsimile terminal if no response signal is detected within a predetermined period of receiving a command therefrom so that the time limits for response set out in Recommendation T.30 are not exceeded at the facsimile terminal.
Another scheme that is used in the Global System for Mobile Communications (GSM) non-transparent facsimile service (GSM 03.46) is blind blocking of retransmitted commands. Here the facsimile adapter blocks re-transmitted commands from reaching the remote end until a response is received from the remote end. This response is then forwarded to the command sending entity which thinks that it is a response to the most recently retransmitted command although it is a delayed response to the first transmitted command. This scheme has the potential danger of timing out at the post-message phase of the facsimile call if too many retransmissions occurred during image transfer phase of facsimile call. WO 95/22224 provides a solution to the long delay problem by using the flag-stuffing, where the HDLC flags are autonomously generated by Intelligent Facsimile Relay (IFR) equivalents. The facsimile interface unit disclosed in WO 95/22224 detects a transmitted signal from a transmitting facsimile apparatus and detects whether a response signal to said transmitted signal is received from another facsimile apparatus within a predetermined period. If no response signal is detected after a predetermined period, a command-repeat (CRP) is transmitted to the command-sending entity. This forces the command sending entity to repeat the previously transmitted command until a response is received from the remote end. While this takes care of the time-out problem of T.30, the problem of collision at the 2-wire link is not solved. This is especially true when there is a long delay in the Public Switch Telephone Network (PSTN) leg of the connection and the delay is unknown. A consequence of this is that the CRP is received in error by the command sending entity.
While blind blocking, flag-stuffing techniques and the like may be used for transparent facsimile services that do not use retransmission of GSM air interface, it poses the danger of loss of the very first transmitted command on the air interface, making the scheme too sensitive to channel impairments. Furthermore, many facsimile machines ignore the first DIS command.
This invention provides a technique that will permit reliable facsimile transmission in presence of long delays. The main advantage of this technique is that it does not require specific knowledge of the delays in the system in order to avoid signal collision and to keep the connection alive.
Essentially the methods and apparatus disclosed in the described embodiments provide monitoring and/or manipulation through the use of entities, referred to as Intelligent Facsimile Relays (IFRs) that are physically located between the two end facsimile machines communicating over a long delay and possibly impaired link. The IFRs constantly monitor (and if necessary manipulate) the ITU-T T.30 protocols in both directions and intelligently decides to transmit, relay, buffer, or discard individual messages in the T.30 protocol. The primary intent here is to detect and avoid signal collisions on two wire interfaces and simultaneously prevent disconnects due to repeated ITU-T T.30 time-outs. In addition to monitoring, whenever it is necessary to manipulate T.30 messages (such as manipulation of user rates in Digital Identification Signal (DIS), intentionally disabling End Communication Message (ECM) mode of operation, etc.), the IFRs perform a forced corruption if the IFRs themselves receive a T.30 message in error. Furthermore, the IFR appropriately generates and transmits standard non-user specific T.30 messages in place of the messages received from the end facsimile machines, thereby preventing the possibility of blindly relaying a possibly corrupted message to the other end. This provides additional robustness to the T.30 protocol handling.
Briefly summarized, the present invention relates to a facsimile transmission relay apparatus for improving call success rates in long delay environments. The apparatus, discloses in the embodiment, herein and intelligent facsimile relay, has a first bi-directional telecommunication interface for receiving a facsimile signal from a transmitting facsimile device via a first network, such as the PSTN. The relay apparatus is provided with a buffer memory for storing the facsimile signals from the transmitting facsimile device, and a second bi-directional telecommunication interface facilitates the retransmission of the facsimile signal via a second network to a receiving facsimile device, e.g., via a wireless telecommunications network. An information processor is coupled to the second interface monitoring messages from the receiving facsimile device and for generating messages to the receiving facsimile device in order to maintain communications from the transmitting facsimile device via the first and second networks.
It will be understood that both the foregoing general description and the following detailed description are exemplary and intended to provide further explanation of the invention as claimed. The accompanying drawings provide an understanding of the invention as described in the preferred embodiments to illustrate the invention and to serve to explain the principles of the invention.